On Aug 23, 10:00*am, John Cochrane
wrote:
Well, just for a contrary opinion, I disagree with Darryl and Kirk. Mc
setting is the right set of units for everything in soaring.

If you must think about glide angles, the right units are D/L not L/
D. *L/D goes through infinity when you run in to lift. D/L (feet per
mile, meters per kilometer) does not. If you gain 200 feet in lift vs.
lose 200 feet in lift, L/D shows radically different changes, D/L does
not.

The "safety profile" for making it to a goal with constant (say 99%)
probability follows a roughly square root function of distance.
(Square root follows if lift/sink are independent over distance) *Most
of us approximate this with a relatively high Mc setting (3-4) plus a
reserve altitude.

Smoother conditions -- less lift or sink -- means less uncertainty
about your glide. So, paradoxically, you can use more aggressive
safety settings if there is no lift around, because then there is no
sink around. Strong lift mans strong sink; half chance of escaping in
10 knots, half chance of hitting the dirt in 10 knot sink. Therefore,
use a higher Mc setting and higher reserve altitude with stronger lift/
sink or general uncertainty.

To fly a safety glide you want to have the glide computer at a high Mc
setting, but fly slowly and accept weaker lift. Many pilots disconnect
the glide computer from the vario for this reason. Well, I do.
Instrument makers should recognize this difference and make it easier
to have a different Mc for glide than vario.

Wind is irrelevant here, with one exception. As you lower the Mc
setting heading upwind, you will discover a point at which lower Mc
setttings seem to make it worse. This is a featuer not a bug. The best
glide in wind occurs at a higher Mc setting. don't fly slower than
that, don't take weaker thermals than that, or you wont get home

John Cochrane

I did a little calculation for a standard Cirrus with no wind. I did
this by using SeeYou mobile in simulator mode. I did manual math for
angle and feet per NM. Here are a few numbers. They might not format
correctly.

Required L/D 38, 28, 20, 15, 10
Required MC 0, 3.5, 7.4, 12, 22
glide angle deg 1.7, 2.2, 2.9, 3.8, 5.7
feet per NM 158, 215, 300, 400, 600

Then I added a big headwind. Required L/D stays the same, but MC
corrects for winds. ... akiley